The present invention relates to a telescopic shaft which can be integrated into the steering column of a motor vehicle, which steering column transmits the turning of the steering wheel to the steering box of the vehicle.
As is well known, steering shafts of motor vehicles are composed, in a general manner, of two portions or sections hinged to one another by way of a Cardan-type joint, comprising specifically a main portion or shaft, at one of the ends of which the steering wheel is fixed, and a secondary portion or shaft hinged to the opposite end of the main shaft and having another end hinged, by way of a Cardan-type joint, to the control kingpin of the steering box.
In the present state of the art, at least one of the shafts has a telescopic structure, comprising at least two tubular members, one of which is fitted inside the other and suitably retained in a specific relative position. The tubular members can be displaced from the specific relative position to thereby reduce the length of the assembly when the assembly is subjected to a predetermined force in the axial direction. This is done in this way mainly for reasons of safety, with a view toward avoiding very serious injuries which the steering column could otherwise cause to the driver or to the passengers of the vehicle, for example, as a result of a frontal collision. However, in the case of the secondary shaft or short portion of the column, the telescopic structure facilitates the mounting of this shaft in the vehicle, avoiding the need to use Cardan forks having an open flange, the disadvantages of which, with respect to the forks having a closed or semi-open mounting neck, are obvious and have already been widely recognized. Accordingly, several models of motor vehicles in which the space for mounting the secondary axle or shaft is particularly small, the shaft is composed, not of two, but of three telescopic elements, whereby the overall length of the shaft can be minimized at the time of mounting.
In the solutions which are presently known, the integral telescopic tubular portions of the shaft fit together by way of a region of non-circular cross section, for example polygonal, which provides a rotational coupling between the tubular portions. In one of these known solutions, the region of non-circular cross section of the male element of the telescopic system has rings of plastic material which are injected thereon, for example two in number, which act as friction elements against an outer female element, causing the relative sliding of these two members to occur only in response to a force of predetermined magnitude (generally between 80 and 200 kg). This solution is relatively simple, given that the rings can be injected directly, once the telescopic system has been mounted, through orifices provided in the female element. This solution, however, has the serious disadvantage of making it enormously difficult to obtain reliable and, above all, constant results, with regard to the magnitude of force which the telescopic system must withstand before a reduction in length occurs. In particular, this force depends upon a series of variables (quality of the plastic material, pressure at which the plastic material is injected, contractions which it experiences since injection, regularity and precision of the section of the two tubular elements, etc.) which are practically uncontrollable. In order to obviate these drawbacks it has already been proposed, according to another known solution, to provide on at least one of the rings of plastic material, projections of the same material which fit into corresponding orifices provided in the female tubular element and retain the two elements in a specific relative position. This solution makes it possible to calculate with an acceptable degree of accuracy the force which the telescopic system must withstand before deformation occurs, that is to say, the shear force necessary to break the projections. This solution, however, has the main disadvantage that, once these projections are broken, there is no longer anything to oppose the relative movement of the two members, so that the assembly no longer develops any braking or damping action. On the other hand, in the majority of known solutions the telescopic shaft can only collapse in response to a force of superior magnitude which, in turn, results from a collision or accident. The telescopic shaft, therefore, does not act like a telescopic shaft during mounting. Instead, during mounting and adjustment, the telescopic shaft acts like a rigid body of invariable length.